1,721,130 research outputs found
NMR/FTIR study on the various siliceous catalysts used in the vapor phase Beckmann rearrangement reaction
No full textSurface properties of VOx-SiO2 and VOx-Al2O3 catalysts: a spectroscopic study by FT-IR, Micro-Raman, XPS and EPR techniques
No full textRecently, the flame pyrolysis method (FP) has been proposed for the preparation of VOx-SiO2 and VOx-Al2O3 catalysts (nominal content of V2O5 5-50 % by weight) effective in the oxidative dehydrogenation (ODH) of propane to propylene [1,2]. As a whole, catalytic tests revealed that VOx-Al2O3 samples were more active whereas VOx-SiO2 exhibited higher selectivity to propylene. Specifically, the most selective catalyst resulted to be that with a 10 % nominal content of V2O5 (V10Si). As expected, the catalytic performances of such catalysts may be correlated to their physico-chemical properties, as studied by FT-IR, Micro-Raman, XPS and EPR techniques. IR spectra of VOx-SiO2 samples (Figure 1) showed a band at 930 cm-1, due to the vibration of SiO44- groups strongly polarized by interaction with vicinal vanadium atoms: such [SiOδ-...Vδ+] species were not observed with a sample prepared by impregnation (V10Si-i), indicating that V-incorporation into the silica framework takes place during FP. No evidence of V introduction in the framework of alumina was instead detected in VOx-Al2O3 systems. Micro-Raman analysis showed the presence of isolated VOx species (signals 1027 and 512 cm-1) only with sample V10Si, whereas with all other VOx-SiO2 and VOx-Al2O3 systems, and with samples prepared by impregnation, only bands of crystalline V2O5 were detected. Adsorption of CO at nominal -196 °C on V10Si sample showed the presence of different OH species, with the following relative acidity scale: isolated SiOH < H-bonded SiOH < V-OH. Likewise, the Brønsted acidity was studied by means of NH3 adsorption on samples outgassed at 500 °C. The blue-shift (ΔνNH4+) of the band due to the bending vibration of NH4+ species with respect to that of free NH4+ (1410 cm-1) can be used as a semi-quantitative measure of the acidic strength of Brønsted sites: the smaller the shift the higher the Brønsted acidity. A good correlation has been observed between the acidic strength of Brønsted sites and the selectivity to propylene for both VOx-SiO2 and VOx-Al2O3 systems: for both sets of catalysts, the selectivity increases with the decreasing of the Brønsted acidic strength, which favors side-reactions catalyzed by acids leading to a higher amount of COx [3]. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of VOx species. Indeed, XPS can provide information on the V oxidation states by considering the V 2p3/2 binding energies (BE). Interestingly, the V10Si sample exhibited a broad peak at higher V 2p3/2 BE as compared to other VOx-SiO2 systems, probably due to the presence of highly dispersed V5+-OH groups partially incorporated into the silica matrix and interacting poorly with one another [4,5]. A similar spectra was recorded on V5Si, although the latter exhibited an additional signal at ca. 516 eV, suggesting also the presence V3+ species at the surface. The latter result is in fair agreement with the worse catalytic performances of V5Si towards the ODH of propane, kinetically modeled through a MvK mechanism based on V5+ reduction and subsequent reoxidation. On the contrary, at higher V-contents and with V10Si-i sample peaks at low V 2p3/2 BE appeared, typically assigned to extra-framework V2O5. EPR spectra revealed weaker V=O bond in VOx- Al2O3 systems: being V=O bond strength an index of oxygen availability, this could explain their higher activity towards this reaction. In conclusion, for FP-prepared catalysts: (i) a better dispersion of the active phase can be obtained with silica rather than with alumina; (ii) selectivity to propylene depends on Brønsted acidity; (iii) stronger Brønsted acidity leads to higher COx production. Moreover, the best catalyst (V10Si) was characterized by highly dispersed (isolated) V species, less acidic Brønsted sites, higher V oxidation states and stronger V=O bonds.
References:
[1] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan, L. Forni, J. Catal. 256 (2008) 45-61.
[2] I. Rossetti, L. Fabbrini, N. Ballarini, C. Oliva , F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, Catal. Tod., 141 (2009) 271-281.
[3] K. Chen, A. Khodakov, J. Yang, A. T. Bell, E. Iglesia, J. Catal. 186 (1999) 325-333.
[4] C. Hess, R. Schlögl, Chem. Phys. Lett. 432 (2006) 139-145.
[5] C. Hess, J. Catal. 248 (2007) 120-123
V-containing SBA-15 and MCF prepared by direct synthesis as catalysts of oxidation reactions
No full textIn the oxidative dehydrogenation (ODH) of propane, the most studied catalysts are vanadium oxides (VOx) supported on different oxides [1], usually prepared by impregnation. Recently, the flame pyrolysis method (FP) has been proposed for the preparation of VOx-SiO2 and VOx-Al2O3 systems (nominal content of V2O5: 5-50 % by weight) [2,3]. Catalytic tests, showed that VOx-Al2O3 systems were more active [3], whereas VOx-SiO2 systems were more selective [2]. The most selective catalysts resulted to be that with a 10% nominal content of V2O5 (V10Si). The catalytic performance of such systems may be correlated to their physico-chemical properties, as evaluated by FT-IR and Micro-Raman spectroscopies. IR spectra of VOx-SiO2 samples showed a band at 930 cm-1, due to the vibration of SiO44- groups strongly polarized by interaction with vicinal vanadium atoms: such [SiOδ- Vδ+] species were not observed with a sample prepared by impregnation (V10Si-i), indicating that V incorporation in the silica framework takes place during FP. No evidence of V incorporation in the framework of alumina was instead detected in VOx-Al2O3 systems.
Micro-Raman analysis showed the presence of catalytically active isolated V=O groups (signals 1027 and 512 cm-1) only with sample V10Si, whereas with all other VOx-SiO2 and VOx-Al2O3 samples only the typical bands of crystalline V2O5 were observed [2,3].
Adsorption of CO at nominal 77 K on V10Si sample showed the presence of different OH species, with the following scale of relative acidity: isolated SiOH < H-bonded SiOH < V-OH [2]. The Brønsted acidity was studied by means of NH3 adsorption on samples outgassed at 500 °C. The blue-shift (ΔνNH4+) of the band due to the bending vibration of NH4+ species with respect to that of free NH4+ (1410 cm-1) can be used as a measure of the acidic strength of Brønsted sites: the smaller the shift the higher the Brønsted acidity. An interesting correlation has been observed between the acidic strength of Brønsted sites and the selectivity to propylene for both VOx-SiO2 and VOx-Al2O3 systems: the Figure reports selectivity to propylene, measured at propane isoconversion, as a function of the Brønsted acidity, measured as ΔνNH4+. For both systems, the selectivity increases with the decreasing of the Brønsted acidic strength, which favors side-reactions catalyzed by acids leading to a higher amount of COx [4].
In particular, Brønsted acidity seems to play an important role in the formation of CO, although also the reaction C + CO2 2CO shall also contribute to its formation [3]. CO is mainly a product of the secondary oxidation of propylene, whereas CO2 forms from primary combustion of propane [4-6]. EPR spectra revealed a weaker V=O bond in VOx- Al2O3 systems [3]: being V=O bond strength an index of oxygen availability, this explains their higher activity. In conclusion, for FP catalysts: (i) a better dispersion of the active phase can be obtained with silica rather than with alumina; (ii) selectivity to propylene depends on Brønsted acidity; (iii) stronger Brønsted acidity leads to higher CO production. Therefore, the best catalyst (V10Si) was characterized by isolated vanadium species, less acidic Brønsted sites and stronger V=O bonds.
References.
1. F. Cavani, N. Ballarini, A. Cericola, Catal. Tod. 127 (2007) 113-131. 2. I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan,L. Forni, J. Catal. 256 (2008) 45-61. 3. I. Rossetti, L. Fabbrini, N. Ballarini, C. Oliva , F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, Catal. Tod., in press, ISSN:0920-5861 DOI:10.1016/j.cattod.2008.05.020. 4. K. Chen, A. Khodakov, J. Yang, A. T. Bell, E. Iglesia, J. Catal. 186 (1999) 325-333. 5. O. R. Evans, A. T. Bell, T. Don Tilley, J. Catal. 226 (2004) 292-300. 6. M. D. Argyle, K. Chen, A. T. Bell, E. Iglesia, J. Catal. 208 (2002) 139-142
The oxidative dehydrogenation of propane over V-containing mesoporous silicas: the effect of vanadium dispersion, surface acidity and support properties on the catalytic activity
No full textAmong the many materials considered for propane ODH, V-containing mesoporous silicas characterized by high surface area, large pores and well-dispersed VOx species prove to be most effective [1,2]. V-based catalysts are usually obtained by impregnation: it has been observed, though, that V-containing mesoporous systems (i.e. V-SBA-15 and V-MCF) prepared by direct synthesis exhibit better V-dispersion and superior catalytic performances in both selective and total oxidation reactions, reactions both based on V reduction and subsequent reoxidation (MvK mechanism) [3,4]. As it is known, isolated V-species are beneficial in terms of propene selectivity, whereas higher activity is usually associated to polyvanadates (up to the formation of VOx monolayer) [1-5].In this work, a V-SBA-15 and a V-MCF sample [3,4] (V cont. 2.5 wt.%) prepared by direct synthesis were tested in propane ODH. Their physico-chemical and catalytic properties were compared with those of materials with the same V-content but prepared by different ways: i) two impregnated samples (V-SBA-15-i and V-MCF-i) and ii) a non-porous sample obtained by flame pyrolysis (V-SiO2), a high temperature synthesis technique allowing high V-dispersion to be achieved [2]. As a whole, both V-SBA-15 and V-MCF exhibited better textural properties (i.e. higher SSA) and higher V dispersion (isolated V-species), as compared to impregnated ones, in which polymeric VOx and micro-crystalline V2O5 were detected by TEM, H
2-TPR micro-Raman and DR UV-Vis [4,5]. On the other hand, both impregnated samples showed higher Brønsted acidity (related to V-OH groups), as shown by IR-spectroscopy of adsorbed CO and NH3
Better catalytic performances in terms of selectivity to propene were achieved with both samples directly synthesized, as consequence of higher V dispersion and lower surface acidity. Brønsted acidity seems indeed to be negatively correlated with the selectivity to propene, since stronger Brønsted acidic sites favor the formation of CO.. In contrast, samples directly synthesized show supported V species that can be converted into Lewis acidic sites by dehydroxylation much more easily than impregnated samples (Figure 1).
x and other by-products. A similar correlation was previously observed with a set of non-porous VOx-SiO2 samples prepared by flame pyrolysis (results to be published). Moreover, higher selectivity to propene was obtained over V-MCF as compared to V-SBA-15 (a similar trend was observed with impregnated ones). This behavior was related to the different porous network of the two catalysts: the structure of MCF, featuring a 3-D network with ultra-large cells (20-40 nm), favors molecular diffusion along each direction, unlike cylindrical channels of SBA-15 (3-6 nm) occurring in one dimension mainly (axial diffusion). Molecules can be much more retained inside channels of SBA-15 (deeper oxidation) as compared to MCF. On the other hand, the non-porous sample, with V-species well dispersed and incorporated into the silica framework, exhibited worse catalytic results than mesoporous samples, confirming the important role of mesoporosity in oxidation processes.
References
[1] F. Cavani, N. Ballarini, E. Cericola, Catal. Today 127 (2007) 113.
[2] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, J. Catal. 256 (2008) 45.
[3] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, L. Gaberova, M. Armandi, E. Garrone, Catal. Today, 176 (2011) 458.
[4] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, M. Armandi, C. Thomas, E. Garrone, Catal. Today doi:10.1016/j.cattod.2011.06.028
[5] H. Dai, A.T. Bell, E. Iglesia, J. Catal 221 (2004) 49
Effect of vanadium dispersion and of support properties on the catalytic activity of V-containing mesoporous silicas
No full textIntroduction
SBA-15 and MCF mesoporous silicas that are characterized by large and uniform pores size, thick walls and high surface area, represent very interesting supports for catalytically active species (e.g. vanadium) allowing a large concentration of accessible, isolated and well defined active sites to be obtained [1]. On the other hand, vanadium-based systems are effective in many oxidation reactions, which can be kinetically modeled through a MvK mechanism.
In this work, two series of highly dispersed vanadium-containing systems (namely V-SBA-15 and V-MCF) were prepared by direct synthesis and tested in the catalytic decomposition of dichloromethane, the most stable chlorinated-alkane.
Physico-chemical and catalytic properties of both V-SBA-15 and V-MCF systems were compared with those of V-containing mesoporous samples prepared by wet impregnation and of a non-porous V-containing silica sample.
Experimental part
V-SBA-15 materials with different V-loadings (0.7-4.8 wt.%) were prepared by direct synthesis as reported in the literature [2]. Likewise, two V-MCF samples (2.0-4.5 wt.% vanadium) were prepared by a new direct synthesis (currently under publication). For comparison, two catalysts were synthesized by impregnation of SBA-15 and MCF supports (referred to as V/SBA(i) and V/MCF(i), respectively). A non-porous sample with 3 wt.% vanadium (denoted as V-Si) was obtained by flame pyrolysis method, according to Ref. [3].
Samples were characterized by means of powders XRD, FESEM and TEM microscopies, N2 sorption isotherms at -196 °C, H2-TPR, EPR, 51 V MAS NMR, DR UV-vis, micro-Raman and FT-IR spectroscopies.
The decomposition of dichloromethane was used as probe reaction for catalytic oxidation of Cl-VOCs. Catalytic measurements were carried out in a fixed-bed reactor under both aerobic and anaerobic conditions at different reaction temperatures (200-500 °C).
Results and discussion
The samples prepared by direct synthesis showed higher specific surface areas (up to 820 m2g-1 for V-SBA-15 and 925 m2g-1 for V-MCF) as compared to impregnated ones (530 m2g-1 for V/SBA-15(i) and 645 m2g-1 for V/MCF(i)), in which a decrease occurred of both surface area and porous volume. Micro-Raman spectroscopy of dehydrated V-SBA-15 and V-MCF samples showed V=O stretching modes of isolated V species in tetrahedral coordination (1035 cm-1), whereas typical bands of polymeric VOx species and micro-crystalline V2O5 were observed with V-impregnated samples. Likewise, TEM images revealed the presence of VOx aggregates only at the surface of impregnated samples. DR UV-vis spectra of dehydrated V-SBA-15 systems showed the occurrence of V5+ species in tetrahedral coordination (absorption band at about 260 nm), whereas upon rehydration such species were converted to octahedral ones by coordination of water molecules (absorption band at about 380 nm). By contrast, a higher amount of octahedral coordinated V species was observed with impregnated samples under dehydrated conditions. Indeed, IR spectroscopy showed that more abundant and acidic sites were obtained by impregnation. H2-TPR analysis showed a much easier V reducibility in both V-SBA-15 and V-MCF samples, due to better V dispersion reached by direct synthesis. Higher catalytic activity for the dichloromethane decomposition was achieved over V-SBA-15 and V-MCF samples directly synthesized with respect to impregnated samples, as consequence of their much higher V dispersion (Fig. 1). Furthermore, better dichloromethane conversions were obtained over V-SBA-15 as compared to V-MCF systems (a similar trend was observed with impregnated samples).
These findings seem to be related to the different mesoporous systems under study: molecules should be more retained within pores of SBA-15 framework as compared to ultra-large pores of three-dimensional MCF systems. On the other hand, a non-porous sample (V-Si), with V species well dispersed and incorporated into the silica framework, exhibited lower activity than mesoporous samples, confirming the important role of mesoporosity in total oxidation reactions.
[1] J.M. Thomas, J.C. Hernandez-Garrido, R. Raja, R.G. Bell, Phys. Chem. Phys. 11 (2009) 2799.
[2] M. Piumetti, B. Bonelli, M. Armandi, L. Gaberova, S. Casale, P. Massiani, E. Garrone, Micropor. Mesopor. Mater. (2010) doi:10.1016/j.micromeso.2010.04.011
[3] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan,L. Forni, J. Catal. 256 (2008) 4
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Spectroscopic study of anatase properties. Part 3. Surface acidity
No full textThe surface acidity of 2 prepns. of anatase was examd. by studying the adsorption of pyridine, CO, and linearly held CO2. Pyridine chemisorbs with high coverages, giving a H-bonded species and 2 types of coordinated base, whereas pyridinium is produced in the presence of a fair amt. of SO42- contaminants arising from the method of prepn. The most strongly held pyridine species coordinates to clusters of Lewis-acidic centers and the frequency of the 8a mode is peculiar to 3 different cluster sizes. CO chemisorbs with low coverages on the most acidic Lewis centers and shows the heterogeneity of these centers. Linear CO2 adsorbs at the sites responsible for CO chemisorption and is sensitive to lateral effects and to polarity of the surface layer
Phisico-chemical properties of V-containing mesoporous silicas prepared by direct synthesis
No full textOzone air pollution and daily mortality in Genoa, Italy between 1993 and 1996.
No full textAbstract
The association between ozone (O3) and daily mortality was investigated in Genoa, an Italian city characterized by a Mediterranean climate and a high prevalence of elderly inhabitants. The O3 effect, adjusted for long time trend, seasonality and weather, was assessed using Poisson regression modelling, allowing for overdispersion and autocorrelation, and expressed as mean variation percent of daily mortality per 50 microg/m3 increase (MV). Significant MVs for overall (+4.0%) and cardiovascular (+7.2%) mortality were detected at 1-day lag. The effects were stronger in the warmer season (May-October). Similar estimates were found after restricting the analyses to the elderly (>or=75 years). Furthermore, in this group, higher MVs were observed for total mortality at 2-day lag. A statistically significant synergistic effect between O3 and temperature was observed for cardiovascular mortality, particularly in elderly people, with an evident increase in mortality risk above 26 degrees C (MV +30.0% for the whole population and +40.0% for the elderly, respectively). This investigation highlights the importance of taking local climatic and demographic features into account when comparing different time-series studies, and substantiates the influence of photochemical pollution on mortality trends in small urban areas
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